Fluid-filled mattresses have been used for many years. Such mattresses generally comprise an outer cover having upper and lower walls connected at their peripheral portions by side and end walls, and with a plurality of vertically extending ribs interposed between the upper and lower walls of the cover. The cover is sealed and contains either air, water or a mixture thereof. In most cases, the cover and ribs are formed of a synthetic plastic material. The upper and lower ends of the ribs are attached to the interior of the upper and lower walls of the cover by dielectric heating to form a bond. It is necessary to heat the bead to a high temperature to obtain a bond. As a result the cover material is degraded. Because of such degradation, the cover material often pulls away from the ribs. It has also been proposed to form horizontal flaps on the upper and lower ends of the ribs and to adhere such flaps to the upper and lower surfaces of the cover, as by dielectric heating. With this construction it is very difficult to adhere the center portion of the ribs to the cover material. Accordingly, failure of the connection between the rib and the cover material can occur.
Failure of the attachment of the ribs to the cover also frequently occurs at the connection between the outer ends of the ribs and the cover. Such failure results from the high stress which occurs at these connection points because the ends of the ribs cannot stretch in a vertical plane.